1. Field of the Invention
This invention relates generally to digital communications, and more specifically relates to the proper determination of transmitted digital data signals, in particular, non-return-to-zero (NRZ) signals.
2. Description of the Prior Art
It is generally known to set the threshold for determining the state of antipodal information symbols based on averaging the received data, a baseband signal, over a relatively long period of time, for example, 200 bit periods or more. The longer the averaging period is, a more precise threshold may be realized. Such a rule may be suitable for many types of modulation, including frequency shift keying (FSK), quadrature phase shift keying (QPSK), offset quadrature phase shift keying (OQPSK) and minimum shift keying (MSK).
Gaussian minimum shift keying (GMSK) and gaussian frequency shift keying (GFSK) modulation techniques are becoming more prevalent in communication systems because of their resultant increased spectral efficiency. Such modulators are implemented by using a premodulation gaussian low-pass filter in the baseband processor of an MSK or FSK modulator, and to which is provided a balanced NRZ data signal. An example of a GMSK modulator is disclosed in U.S. Pat. No. 4,567,602 to Kamilo Feher.
One of the problems associated with use of a gaussian low-pass pre-filter is that it has a "memory" or energy storage effect such that the swing and, thus, the negative and positive amplitudes of the NRZ received data signal are affected. The effect of the gaussian pre-filter in a GMSK system is shown in FIG. 1. The swing and peak amplitudes of the transmitted signal are clearly affected particularly when successive bits alternate in sign (i.e., polarity).
Over long periods of time, this should not have a deleterious effect on the threshold determination, conventionally done by averaging consecutive bits of the received data stream. But for shorter averaging periods, where the probability of equal antipodal excursions of data decreases, and the probability of equal antipodal excursions of data pattern decrease further, the bit determination threshold may be significantly affected. Thus, the gaussian pre-filter in a GMSK or GFSK modem further limits the ability to accurately determine the threshold over fewer bit periods than is conventionally done.